@Sejisensei
Ha, I like 'Prism' also quite a bit, it's a good song.
@JRendell
I immediately thought of Kokia too, liked her 'Red Garden' songs.
Then there is also Kanon, with the ED of 'Guin Saga' e.g.
Not to forget Ayaka Hirahara
Mika Arisaka did a good job on her Twelwe Kingdoms songs too,
as did Houko Kuwashima.
These are the ones I remember atm.
EDIT: Forgot this one, sung in English.
EDIT2 (since we are not restricted to anime-related music): I absolutely adore her voice.